1. Field of the Invention
The present invention relates to the field of DC power supplies, and more particularly, to the field of regulated DC power supplies.
2. Background Information
Over the years, power supplies providing a DC output voltage have progressed from rather crude systems relying on selenium rectifiers or semiconductor diodes for their rectification which, although effective, were relatively inefficient and relatively poorly regulated by current standards, to much more sophisticated systems including sophisticated voltage regulation systems for providing tightly controlled output voltages which may have specifications as tight as 1%.
There are many systems in which a tightly regulated main power supply voltage must be supplemented by one or more auxiliary power supply voltages at other voltages. There are a number of ways of providing these auxiliary voltages.
In one of these, a resistive voltage divider (often including a zener diode) provides the auxiliary voltages. Such systems provide limited regulation of the auxiliary voltages and are quite lossy and thus are used primarily for low power outputs.
When overall power supply system efficiencies were not a major concern as was the case with benchtop power supplies for laboratory use, regulation of output voltages could be accomplished through use of zener diodes connected across the output to limit the maximum output voltage. Such systems were particularly common where the output voltage had a maximum limit on it, but a relatively loose minimum requirement.
With another alternative, a separate voltage supply is used for each output voltage. Such systems offer the advantage of tight control over all voltages. However, such systems suffer from several disadvantages. First, multiple transformers and regulation systems must be provided to control the various output voltages. This is expensive initially and results in a more lossy or lower efficiency system than some other techniques. Further, in some systems, the auxiliary voltages must have specified relationships to the main voltage. In systems with totally separate power supplies, this requires cross-connection of regulation systems to ensure that the output voltages track in the specified manner.
A further, frequently selected, alternative is to employ a single transformer with multiple secondary windings to provide these different voltages. In such systems, each distinct output voltage is derived from a different secondary winding of the transformer. Initially, such designs were quite effective because system specifications normally provided a much looser specification on the auxiliary voltages than on the main or master output voltage. As a result, normally no regulation was required on the individual auxiliary voltages. As electronic systems have become more complex, specifications on auxiliary output voltages for power supplies have become substantially tighter with the result that regulation requirements may be as tight as 1%.
For a number of reasons, including overall power consumption, thermal dissipation and a desire for high density packaging, modern power supply requirements often include tight efficiency specifications which may be as high as 80 or 90% overall efficiency. Consequently, such brute force auxiliary regulation techniques as the use of zener diodes across the auxiliary output are no longer considered acceptable. Similarly, multiple, essentially separate, power supplies are not an acceptable solution. Consequently, a need has developed for auxiliary regulation systems which provide tight regulation of their output voltages in combination with high efficiency operation in power supplies in which a single transformer drives all of the outputs.
Many modern power supply regulation systems rely on control of the frequency or pulse width of the signal provided to the primary of the power supply transformer to regulate the master output voltage of the power supply. In such power supplies, a change in the load current being drawn from the output (or the main output if there is more than one) produces a change in the signal applied to the primary which causes a corresponding change in the power available at the master output terminal. In the absence of auxiliary regulation, this causes a corresponding change (increase or decrease) in the power provided to the auxiliary output and thus, for a constant auxiliary load, results in a change in voltage at that auxiliary output.
In an article entitled "Post-Regulation Techniques for 100 KHz to 300 KHz Multiple-Output PWM Supplies (Limitations, Trends and Predictions)" given at the 1989 High Frequency Power Conversion International Conference, Clifford L. Jamerson, provides an overview of the considerations in the selection of a regulation technique for a multiple output power supply. He also provides brief descriptions and synopses of the advantages and disadvantages of 22 different techniques for regulating the auxiliary outputs of such a power supply. In this article, he indicates that the use of linear pass regulators is and will be high for supplies providing full load current outputs in the range from 0.1 to 2 amperes, square loop mag amp usage is high and will continue to be high for supplies having full load outputs of 2 to 20 amperes and synchronous switch post regulator (SSPR) usage, although not yet high, will become high for outputs having full load currents in the range from 2 to 10 amperes.
At pages 270-271 of that article, Mr. Jamerson sets forth advantages and disadvantages of synchronous switch post regulation. A disadvantage of SSPR which is not mentioned in that list is the fact that either one half of or all of the full load current flows through the switching device (depending on its location) and thus, the switching device contributes significantly to the overall loss in the system, via its conduction losses in addition to any switching losses.
As a consequence of modern power supply system specifications, there is a need for auxiliary output power supply regulation systems which provide tight regulation with high efficiency when operating in conjunction with separate regulation of a master output via control of the signal applied to the transformer primary. Further, a regulation technique which is advantageous over a wide range from low current (in the vicinity of 0.1 amp) to high currents (of 20 amps or more) would be desirable.